1. Field of the Invention
The present invention relates to a subassembly and an optical module that are ideal in applications in optical communication systems and the like.
2. Description of the Related Art
An optical module in the related art is normally achieved by packaging a subassembly substrate at which a laser diode is mounted, a lens provided to achieve optical coupling and an optical fiber. The lens is constituted of a ball lens or an aspherical lens in most cases. Coaxial optical modules known in the related art include can-type optical modules, pigtail-type optical modules (see, for instance, Japanese Patent Laid-open Publication No. 5-343709) and receptacle-type optical modules. The optical axes are adjusted for alignment by utilizing an aligning device while monitoring the light output.
The need for optical modules achieving a higher degree of coupling efficiency that can be, at the same time, offered at lower prices have risen in recent years. The semiconductor laser module disclosed in Japanese Patent Laid-open Publication No. 8-23138, for instance, is manufactured by using fewer parts and through a simpler alignment process in order to achieve a cost reduction. More specifically, a holder having a ball lens attached thereto is utilized, and the holder and the laser package are fixed. After a ferrule in which an optical fiber is housed is set against the holder, the ferrule is aligned along directions perpendicular to the optical axis.
In addition, an optical module equipped with an isolator has been often used in recent years to prevent return light from entering the laser diode. The isolator may be constituted mainly with a Faraday rotator and polarizers mounted on the two sides of the Faraday rotator, for instance. In some cases, magnets that apply magnetic fields to the Faraday rotator are disposed around the Faraday rotator and the polarizers. An optical module equipped with an isolator usually adopts a structure in which the isolator is located toward the optical fiber end with a small light flux diameter so as to acheive miniaturization of the isolator (see, for instance, U.S. Pat. No. 5,841,922). The miniaturization of the isolator is a crucial priority since it greatly contributes to lowering the cost of the optical module and also helps to achieve miniaturization of the optical module itself.